I/O Wiring

One of the many advantages to using a PLC/PAC is
the simplicity of the I/O wiring.

I/O devices are wired to I/O points on a fixed I/O
unit and to I/O modules in a modular unit. Input
devices such as switches, pushbuttons and sensors
are wired to input module points and output
devices such as indicator lights, solenoids and
motor starter coils are wired to output module points.

Note: For this tutorial we will be referring to
modular PLC/PAC's.

I/O modules can accept ACV, DCV or a combination of
ACV and DCV. Most modern automation systems use
DCV I/O, therefore we will focus on DCV.

DC voltage has a polarity. We already know that current
flow is from negative to positive. This current flow is
also referred to as electron flow and is also the
current that is measured when making measurements
with a meter.

There is also another current flow. This current flow
is called 'conventional current' and is the current
flow that solid state physis use. This current flows
from positive to negative. When electrons flow from
one atom to another they leave a hole behind on the
atom that they leave. If you could watch the
electrons moving from atom to atom, the electrons
would be moving in one direction (negive to positive)
and the holes would look like they are moving in the
opposite direction (positive to negative).

DCV I/O module polarity is referenced using the terms
sink and source. The terms are derived from
conventional current flow, but we will explain sink
and source in terms of electron flow.

Why should we care what direction current
is flowing? Mechanical switches, indicator lamps and
solenoids don't care what polarity the current is
flowing. But sensors such as photoelectric,
inductive and capacitive proximity sensors are
also wired to PLC/PAC I/O modules. These sensors
are solid state, using diodes and transistors
in their internal workings. These devices require
current to be flowing in the right direction.

Definition: Sink/SourceClick the terms for details
and wiring diagram examples).

A sink I/O device or I/O module will always
have a connection to the negative side of
the DC power supply. The negative side of
the DC power supply is referred to by any
one of or either of the terms: ground,
common, DC common, return, DC return, etc.
All these terms refer to the same electrical
point; the negative side of the DC power
power supply.